Magnetic memory device and method of manufacturing the same

ABSTRACT

A magnetic memory device including a memory layer having a vertical magnetization on the layer surface, of which the direction of magnetization is changed according to information; and a reference layer provided against the memory layer, and being a basis of information while having a vertical magnetization on the layer surface, wherein the memory device memorizes the information by reversing the magnetization of the memory layer by a spin torque generated when a current flows between layers made from the memory layer, the nonmagnetization layer and the reference layer, and a coercive force of the memory layer at a memorization temperature is 0.7 times or less than a coercive force at room temperature, and a heat conductivity of a center portion of an electrode formed on one side of the memory layer in the direction of the layer surface is lower than a heat conductivity of surroundings thereof.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.13/192995 filed Jul. 28, 2011, the entirety of which is incorporatedherein by reference to the extent permitted by law. The presentapplication claims the benefit of priority to Japanese PatentApplication No. JP 2010-177104 filed on Aug. 6, 2010 in the Japan PatentOffice, the entirety of which is incorporated by reference herein to theextent permitted by law.

BACKGROUND

The present disclosure relates to a magnetic memory device which islaminated by a plurality of vertical magnetization layers including amemory layer and a reference layer through a nonmagnetic substance, andmemorizes information by reverse magnetization in a spin torquegenerated when a current flows between these layers, and a method ofmanufacturing the magnetic memory device.

In information devices such as computers, high density DRAM (DynamicRandom Access Memory) operating at high speed is widely used as a RAM(Random Access Memory). However, since DRAM is a volatile memory inwhich the information disappears when power is turned off, a nonvolatilememory from which the information does not disappear is necessary.

As a nonvolatile memory candidate, MRAM (Magnetic Random Access Memory)memorizing information by magnetizing magnetic material has attractedattention and is undergoing development.

As a method of memorizing MRAM memorization, there is a method ofreversing the magnetization by a current magnetic field, for example, asdisclosed in Japanese Unexamined Patent Application Publication No.2004-193595, involving inducing a magnetization reversal by injectingspin polarized electrons into a memory layer directly. In particular,spin injection magnetization reversal, which can reduce current formemorization and reduce the size of the device, has received attention.

Furthermore, in order to refine the device, as disclosed in JapaneseUnexamined Patent Application Publication No. 2009-81215, for example, amethod for utilizing a vertically magnetized layer which enables themagnetization direction of a magnetic material to become vertical isbeing studied.

SUMMARY

However, in order to realize an even higher density magnetic memory, amagnetic memory device operable at high speed with lower current isnecessary.

Since the disclosure is based on recognition of this fact, it isdesirable to realize a magnetic memory device operable at high speedwith low current.

A magnetic memory device according to the present disclosure includes amemory layer having a vertical magnetization on the layer surface andchanging the direction of the magnetization of the memory layeraccording to information, and a reference layer which is providedagainst the memory layer through a nonmagnetization layer and which is abasis of information while having vertical magnetization on the layersurface, wherein the memory device memorizes the information byreversing the magnetization of the memory layer by a spin torquegenerated when a current flows between layers made from the memorylayer, the nonmagnetization layer, and the reference layer. Furthermore,a coercive force of the memory layer at a memorization temperature isequal to or less than 0.7 times a coercive force at room temperature,and the heat conductivity of a center portion of the electrode formed onone side of the memory layer in the direction of the layer surface islower than the heat conductivity of its surroundings.

For example, the electrode is formed to substantially have a concavecross section, the thickness of the center portion of the electrode isthinner than that of the surroundings of the electrode, and an insulatorof low heat conductivity is filled into a depressed portion formed atthe center portion.

Alternatively, the electrode has a tube-like shape, and an inner part ofthe tube-like shape is filled with an insulator of low heatconductivity.

A method of manufacturing a magnetic memory device of the presentdisclosure includes forming a structure of layers of at least thereference layer, the nonmagnetization layer, and the memory layer on oneside of the electrode of the reference layer, wherein a coercive forceof the memory layer at a memorization temperature is 0.7 times or lessthan a coercive force at room temperature. Furthermore, the methodincludes forming another electrode on one side of the memory layer,wherein this another electrode is filled with an insulator of low heatconductivity, and the heat conductivity of a center portion of theelectrode in the layer surface direction is lower than the heatconductivity of its surroundings.

As the magnetic memory device, although implementing memorization by useof a spin torque caused by spin injection magnetization reversal methodcan reduce the current during memorization, there is a limit on reducingcurrent with this alone. Therefore, it is desirable to enable thecurrent for magnetization reversal to be reduced by using heat inmemorization effectively.

A demagnetizing field of the vertical magnetization layer whichconfigures the memory layer is strong at the center portion in thedirection of the layer surface. Also, magnetization reversal happenseasily at the central part.

Therefore, in the electrode of the memory layer side, the heatconductivity is set to be lower at the center portion thereof in thedirection of the layer surface rather than at the surroundings thereof.For example, it is preferable to facilitate the temperature increase byarranging a material of low heat conductivity. Therefore, it is possibleto effectively raise the temperature at the center portion of the layersurface of a memory layer, reduce the voltage for magnetizationreversal, and reduce current for memorization and time for memorization.

In particular, this effect is obtainable to a remarkable extent when thestructure of the memory layer is formed such that a coercive force ofthe memory layer at a memorization temperature (about 200° C.) is 0.7times or less than that at room temperature (for example, 23° C.)

It is possible to realize a nonvolatile memory operable at high speedwith low current by the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a magnetic memory in which a magneticmemory device is used according to the embodiment of the presentdisclosure.

FIGS. 2A and 2B are a view illustrating the structure of a magneticmemory device according to the embodiment.

FIGS. 3A and 3B are a view illustrating the structure of a generalmagnetic memory device as a comparative example.

FIGS. 4A to 4D are a view illustrating a first structure example and theprocess sequence of the magnetic memory device according to theembodiment.

FIGS. 5A to 5E are a view illustrating a second structure example andthe process sequence of the magnetic memory device according to theembodiment.

FIGS. 6A to 6E are a view illustrating a third structure example and theprocess sequence of the magnetic memory device according to theembodiment.

FIGS. 7A to 7F are a view illustrating a fourth structure example andthe process sequence of the magnetic memory device according to theembodiment.

FIGS. 8A and 8B are a view illustrating a specific example of themagnetic memory device structure according to the embodiment.

FIGS. 9A and 9B are a view illustrating calculated results oftemperature distribution, and the relationship of a ratio of coerciveforce and reversal voltage at temperatures of 200° C. and 23° C.according to the embodiment and the comparative example.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the disclosure are described in thefollowing order.

-   <1. Overview of a magnetic memory structure>-   <2. One example of the structure of a magnetic memory device of an    embodiment >-   <3. The first example of the structure and a manufacturing order>-   <4. The second example of the structure and a manufacturing order>-   <5. The third example of the structure and a manufacturing order>-   <6. The fourth example of the structure and a manufacturing order>-   <7. Embodiment>

1. Overview of a Magnetic Memory Structure

First, the magnetic memory structure which is used as the magneticmemory device according to an embodiment of the present disclosure willbe explained. FIG. 1 schematically shows an overview of the structure ofthe magnetic memory.

The magnetic memory 10 includes two types of address wiringsintersecting each other, for example, a word line and a bit line, andthe magnetic memory device 1 is located in the vicinity of theintersection between the two types of address wirings. The magneticmemory device 1 includes a structure as an embodiment described below.

In a magnetic memory 10, a drain area 8, a source area 7, and a gateelectrode 3 configuring a transistor for selecting the correspondingmemory device 1 are formed individually in a region separated by adevice separating layer 2 of a semiconductor substrate, such as, forexample, Si, or the like.

The gate electrode 3 also has a role as the wiring for addressing (forexample, word line) which is elongated forward and backward in FIG. 1.

The drain area 8 is commonly formed between transistors for selectingright and left in FIG. 1, and this drain area 8 is connected with thewiring 9.

The magnetic memory device 1 is also arranged between the source region7 and the other side of wiring 6 (for example, bit line) for addressingwhich is extended to the left and right in FIG. 1 disposed on upperside. The magnetic memory device 1 includes a memory layer which has thevertical magnetization and made of a ferromagnetic layer, themagnetization direction of which is reversed by a spin injection, and areference layer having the magnetization direction which is a referencefor information memorized in the memory layer.

Also, the magnetic memory device 1 is located in the vicinity ofintersection between the gate electrode 3 and the wiring 6 which are twotypes of address lines, and connected with the upper and lower contactlayers 4. Therefore, it is possible to conduct electric current up anddown to the magnetic memory device 1 through two types of lines, inother words, the gate electrode 3 and the wiring 6, and, by spininjection, it is possible to enable reversing of the magnetizationdirection of the memory layer corresponding to the information.

On the other hand, the structure shown in FIG. 1 is only one example forexplaining the magnetic memory. Therefore, for example, it is possibleto form the wiring 6 without providing the contact layer 4 on themagnetic memory device 1.

2. One Example of the Structure of a Magnetic Memory Device of anEmbodiment

As illustrated above, according to the disclosure, it is possible tooperate the magnetic memory device using a vertical magnetization layerat high speed with a lower current.

The inventors of the present disclosure carried out varied research toaccomplish above matters, and as a result found that it is preferable toform the magnetic memory device having the reference layer and thememory layer having a vertical magnetization and laminated through anonmagnetization layer as follows.

In other words, while making a coercive force of the memory layer at amemorization temperature (about 200° C.) 0.7 times or less than acoercive force at room temperature (about 23° C.), the structure andmaterial of the electrode has a property in which the heat conductivityof a center portion of the electrode is lower than the heat conductivityof its surroundings seen from the layer surface direction of the memorylayer.

Then, because of the heat generated during a memorization process, thetemperature at the center portion of the device becomes significantlyhigher than the temperature at the surroundings of the device so that areverse magnetic area which causes reverse magnetization at the centerportion of the device is likely to be formed. Therefore, a magneticmemory device capable of memorization at high speed with a lower currentis realized.

As a simple structure capable of forming the above mentioned temperaturedistribution, a metal layer is formed to be thinner than the depth ofthe hollow in the hollow which is formed in the insulator separating theupper and lower electrodes of the magnetic memory device in order totake conductivity with the magnetic memory device, and an insulator oflow heat conductivity is provided to the center portion where there isno metal to fill the hollow.

Because of this structure, the heat conductivity of the center portionof the memory layer declines and the heat conductivity of thesurrounding rises, so that the temperature of the center portion islikely to rise during memorization.

Also, in order to augment the difference of heat conductivity, byremoving at least one portion of the metal layer formed at the bottom ofthe hollow of the metals, making the layer thinner, or setting aside theperiphery of the hollow, and thereby forming a cylindrical electrode, itis possible to generate a greater temperature difference in the memorylayer.

The metals used as the electrode of the magnetic memory device of thepresent disclosure are preferably Cu and Al or the like with a high heatconductivity, but W and tantalum or the like with a slightly low heatconductivity may be sufficient. As an insulator with low heatconductivity, porous silicon oxide, organic matter or the like isapplied, however, general silicon oxide or the like can be used.

FIGS. 2A and 2B are a structure example of a magnetic memory device 1according to the embodiment of the disclosure. FIG. 2A shows themagnetic memory device having a cylindrical shape, FIG. 2B shows thecross section thereof.

One structure example of a general magnetic memory device 100 is shownin FIGS. 3A and 3B similarly to FIGS. 2A and 2B in order to allowcomparison.

As shown in FIGS. 2A and 2B, the magnetic memory device 1 with verticalmagnetization according to an embodiment includes at least a referencelayer (magnetization fixed layer) 12, a nonmagnetization (tunnel barrierlayer) 13, and a memory layer 14, laminated between a lower electrode 11and an upper electrode 15.

As shown in FIG. 2B, a wiring 6 is formed on top of the upper electrode15.

In FIG. 2A, the arrows in the memory layer 14 and the reference layer 12denote the magnetization direction, and the arrow of the memory layer 14denotes that the vertical magnetization direction is reversed by spininjection.

The memory layer 14 is a ferromagnet having a magnetic moment, themagnetization direction of which freely changes vertically against thelayer surface.

The reference layer 12 is a ferromagnet having a magnetic moment, themagnetization of which is fixed to the vertical direction against thelayer surface.

The process of memorizing information is performed by the direction ofmagnetization of the memory layer 14 which has a property of single-axisanisotropy. The write operation is performed by applying a current tothe direction vertical against the layer surface and generating spintorque magnetization reversal. In this way, since the reference layer 12is provided as a magnetization fixed layer provided at an underlayerwith respect to the memory layer 14 of which the magnetization directionis reversed by spin injection, the reference layer 12 is considered as areference for the memory information (magnetization direction) of thememory layer 14.

The memory layer 14 includes magnetic materials which have a property ofperpendicular anisotropy. These magnetic materials include rare earthelement -transition metal alloys such as TbCoFe, metal multi-layers suchas Co/Pd multi-layers, and ordered alloys such as FePt.

Also, it is preferable to use MgO as a nonmagnetization layer (tunnelbarrier layer) 13 in order to realize a high magnetoresistance changeratio which provides a large read-out signal in the spin injectionmagnetic memory device 1.

A magnetic layer with a high reversion current is used as the referencelayer 12. A high performance memory device is achieved by using amagnetic layer having a higher reversion current than the memory layer14.

For example, an alloy with the main component Co, which includes atleast one of Cr, Ta, Nb, V, W, Hf, Ti, Zr, Pt, Pd, Fe, and Ni, is usedas the reference layer 12. For example, it is possible to use CoCr,CoPt, CoCrTa, CoCrPt, and the like. Also, it is possible to useamorphous alloys of Tb, Dy, Gd and transition metals. For example, it ispossible to use TbFe, TbCo, TbFeCo, and the like.

Also, the reference layer 12 can be formed by only a ferromagneticlayer, and be formed as well by a laminated ferromagnetic structurelaminated by a number of ferromagnetic layers through a tunnel barrierlayer.

The basic layer structure explained above is the same as the generalmagnetic memory device 100, as can be seen by referring to FIGS. 3A and3B.

The magnetic memory device 1 of the present embodiment is different fromthe general magnetic memory device 100 in FIGS. 3A and 3B in that a lowheat conductivity part 18 is formed in the internal part of the upperelectrode 15.

For example, in FIG. 2B, the thickness of the cross section of thecentral part of the upper electrode 15 in the direction of the layersurface is thinner than that of the circumference part of the upperelectrode 15 so that the cross section is approximately a concave shape,and an insulator of low heat conductivity is filled into the depressedportion formed in the center portion so as to form the low heatconductivity part 18.

By the low heat conductivity part 18, it is possible to set the heatconductivity of surroundings seen from the layer surface direction ofthe memory layer 14 having a circular layer surface high and set theheat conductivity of the center portion low. Then, because of the heatgenerated during the memorization process, the temperature of thecentral part of the device becomes much higher than that of thecircumference part of the device, whereby it becomes easy to form areverse magnetic area which will be a cause of reverse magnetization inthe central part of the device.

Also, although the reference layer 12 is located under the memory layer14 in FIGS. 2A and 2B, it does not matter whether the memory layer 14 islocated under the reference 12 or vice versa.

Also, the shape of the memory layer 14 of the magnetic memory device 1having a memory layer 14 with vertical magnetization may preferably be acylindrical shape or a cone-like shape, and a shape such as a cylindroidor elliptical cone-like shape with low aspect ratio is preferable.

Although it is preferable to form the shape of the reference layer 12with the same shape as the memory layer, the shape is not important ifthe reference layer 12 is under and larger than the memory layer 14.

Also, usually, as shown in FIG. 2B, the surroundings of the magneticmemory device 1 are filled up with the insulator 16.

FIGS. 3A and 3B show a method for manufacturing a magnetic memory device100 including the reference layer 12, nonmagnetization layer 13, thememory layer 14, and electrode material, which are laminated andcomponents of the magnetic memory device 100. Next, after coating aphotoresist, a resist of a device shape is left, and the shape of thedevice is formed under the resist by ion milling or reactive ionetching. Then, the insulating layer 16 is formed, a polishing process isconducted until the upper electrode 15 is exposed, and wiring 17 isformed.

As shown in FIG. 3B, because the upper electrode 15 is uniformly formedon the memory layer 14 in a general magnetic memory device 100, the heatgenerated at the memory layer diffuses quickly through the upperelectrode so that the temperature distribution in the memory layer isnearly equal.

As illustrated in FIGS. 2A and 2B, in the magnetic memory device 1 ofthe present embodiment, as described above, the upper electrode 15 has aconcave like cross section and the depressed portion is filled with alow heat conductivity part 18 so that the temperature of the centerportion seen from the layer surface direction of the memory layer 14 isaccelerated.

3. The First Example of the Structure and a Manufacturing Order

If it is supposed that the structure example of the magnetic memorydevice 1 of the embodiment shown in FIGS. 2A and 2B is a first structureexample, one example of a method for manufacturing the magnetic memorydevice 1 is shown in FIGS. 4A to 4D.

First of all, FIG. 4A indicates a state formed with the lower electrode11, the reference layer 12, the nonmagnetization layer 13, the memorylayer 14, the upper electrode 15, and insulator 16 by the same processsuch as the general magnetic memory device 100 in FIG. 3B.

In other words, the reference layer 12, the nonmagnetization layer 13,the memory layer 14, and electrode material are laminated on the lowerelectrode 11. After that, a photoresist is coated along the areacovering the device, and the shape of the device is formed under theresist by ion milling or reactive ion etching. Next, after providinginsulator material, polishing is conducted until the upper electrode 15is exposed so that a layer forming state is obtained in which thesurroundings are made of an insulating layer 16 and the upper electrode15 is filled therein as shown in FIG. 4A.

Then, from the state of FIG. 4A, the upper electrode 15 is removed asshown in FIG. 4B by selective etching on the material of the upperelectrode 15.

Next, as shown in FIG. 4C, a layer of electrode material 15A which isthe new upper electrode 15 on the upper surface is formed, and also alayer of a low heat conductivity material 18A is formed.

In order to satisfactorily deposit the material even on the wall in thehollow formed after removal of the upper electrode 15 as shown in FIG.4B, the forming method for these is preferably CVD (Chemical VaporDeposition), a bias sputtering method, or the like.

Then, the layer formed on the outside (the upper side) of the hollow isremoved. Therefore, it becomes like FIG. 4D, in other words, themagnetic memory device 1 is formed having the upper electrode 15 with aconcave cross section and having the low heat conductivity part 18filled with the low heat conductivity material 18A in the depressedportion of the concave shape.

Then, the wiring 6 is formed so that a state like FIG. 2B isaccomplished.

4. The Second Example of the Structure and a Manufacturing Order

As another embodiment, a second structure example and the manufacturingprocess will be explained with reference to FIGS. 5A to 5E.

With regard to the shape of the magnetic memory device 1 in FIGS. 4A to4D, although this is a case in which the width of the column is constantin respect to the height, a case in which the width is narrowed incontrast to the height like a cone is easily fabricated. One example asthe structure of the device with a cone-like shape is provided in FIGS.5A to 5E.

FIG. 5A shows a cross section of a magnetic memory device formed in sucha cone-like shape. The reference layer 12, the nonmagnetization layer13, the memory layer 14, and the upper electrode 15 are formed in acone-like shape on the lower electrode 11 and the insulating layer 16 isformed on surroundings thereof. In this step, it can be said that ageneral magnetic memory device with cone like shape is formed.

In the example of the magnetic memory device in FIG. 5A, the material ofthe electrode is etched from above with an ion beam having highlinearity and highly selective etching. Accordingly, as shown in FIG.5B, part of the upper electrode 15 remains in order to make acylindrical hollow. In other words, a hollow with a cylindrical shape isformed.

After that, a low heat conductivity material 18A is filled therein asshown in FIG. 5C.

Then, if it is polished until it is thinner than the state of FIG. 5A,part of the upper electrode 15 is exposed on the surface as shown inFIG. 5D.

In the magnetic memory device 1 shown in FIG. 5D, the upper electrode 15is formed in a pipe shape, and a low heat conductivity part 18 filledwith an insulator of low heat conductivity is formed in the pipe shape.

As shown in FIG. 5E, after forming the magnetic memory device 1 likethis, if the wiring 6 is provided on the surface, the heat conductivityat the surroundings in the direction of the layer surface of the memorylayer 14 improves, and the heat conductivity at the center portiondeteriorates.

5. The Third Example of the Structure and a Manufacturing Order

A third structure example capable of simplifying the making of themagnetic memory device 1 and the manufacturing process will be explainedwith reference to FIGS. 6A to 6E.

FIG. 6A shows the same state of FIG. 4A. However, it is preferable tomake the etching rate of the electrode material of the upper electrode15 higher than that of the surrounding insulator.

If the device in FIG. 6A is etched obliquely from the top in a diagonaldirection by ion milling or the like, a shape with a depressed portionin a center portion is formed as shown in FIG. 6B because of sputteringor reattachment of constituent devices.

The material 18A with a low heat conductivity is filled in thisdepressed portion as shown in FIG. 6C.

Then, if polishing is conducted till the insulating layer 16 becomesthinner than the state of FIG. 6A, part of the upper electrode 15 isexposed on the surface as shown in FIG. 6D.

In the magnetic memory device 1 shown in FIG. 6D, the cross section ofthe upper electrode 15 is substantially a concave shape, and a low heatconductivity part 18 which is filled by an insulator with low heatconductivity is formed in the depressed portion of the concave-likeshape.

After forming the magnetic memory device 1, the wiring 6 is provided onthe surface as shown in FIG. 6E.

In this case, it is possible to improve the heat conductivity at thesurroundings in the direction of layer surface of the memory layer 14,and allow the heat conductivity at the center portion to deteriorate.

6. The Fourth Example of the Structure and a Manufacturing Order

Hereinafter, a fourth structure example and the manufacturing processwill be explained with reference to FIGS. 7A to 7F.

This is an example of the forming of the low heat conductivity part 18in the forming process of the upper electrode 15.

FIG. 7A shows the state in which the reference layer 12, thenonmagnetization layer 13, the memory layer 14 and the low heatconductivity material 18A are laminated sequentially on the lowerelectrode 11. Here, a mask 20 for etching is formed in order to form acolumn of low heat conductivity material 18A.

Then, a column of low heat conductivity material 18A is formed as shownin FIG. 7B by etching under a condition for selectively etching the lowheat conductivity material 18A.

Next, an electrode material (metal layer) 15A which will be an upperelectrode 15 is similarly formed as shown in FIG. 7C. Here, the formedmetal layer is preferably one having a good heat conductivity and beingdifficult to etch under specific conditions, and W, Mo, Ru, Rh, Ir, orthe like may be used.

Then, as shown in FIG. 7D, only the electrode material 15A on thesurroundings of the low heat conductivity material 18A having a columnlike shape remains from the top of the device, and etching is performedso that at least the necessary portion of the memory layer 14 remains.

Then, after forming insulating layer 16 by applying insulating materialaround the device, a planarization process is performed on the uppersurface to attain the state shown in FIG. 7E.

In the magnetic memory device 1 shown in FIG. 7E, the upper electrode 15is formed as a tube shape, and the low heat conductivity part 18 filledby an insulator of low heat conductivity is formed in the inner part ofthe tube.

After forming the magnetic memory device 1 like this, the wiring 6 isformed on the upper surface as shown in FIG. 7F. Therefore, it ispossible to manufacture a magnetic memory device 1 having superior heatconductivity at the surroundings of the memory layer 14 in the directionof the layer surface and having inferior heat conductivity at the centerportion.

7. Embodiment

For example, in each example explained above, with respect to the memorylayer 14, it is possible to obtain an upper electrode structure withhigh heat conductivity at the center portion in the direction of thelayer surface, and promote the temperature rise at the center portion ofthe memory layer 14 at the time of memorization.

For example, an embodiment of the present disclosure when adopting theprocess of FIGS. 6A to 6E will be explained below.

FIG. 8A schematically shows a layer structure of the embodiment.

First, a 5 nm Ta layer 21, which will be one part of the lower electrode11 and functions as a protective layer, is provided on the lowerelectrode 11 made of W (tungsten). A 5 nm Ru layer 22 which is a baselayer thereon in provided.

Then, as a reference layer 12, a 2 nm CoPt layer and a 1 nm CoFeB layerare provided.

Also, MgO with a thickness of 0.8 nm is formed to make anonmagnetization layer 13.

The memory layer 14 is an alternately laminated layer of a 1 nm of CoFeBand Co/Pd. For example, as shown in FIG. 8B, in the memory layer 14, Colayers and Pd layers are alternately laminated on the upper surface ofthe CoFeB layer. Each sample shown in the following FIG. 9B shows anexperimental case when the thickness of each layer of the alternatelylaminated Co/Pd and the total thickness thereof are changed in order toadjust the coercive force of the memory layer 14.

The 5 nm Ta layer 23 which is one part of the upper electrode and has afunction as a protective layer is provided on the top surface of thememory layer 14, and a W layer is formed as the upper electrode 15.

Here, as explained in FIGS. 6A to 6E, the upper electrode 15 of a Wlayer is formed by etching the electrode layer with ion milling from thesurroundings, so the cross section is substantially concave and thedepressed portion is a low heat conductivity part 18.

The magnetic memory device 1 has a size of 150 nm in diameter, thethickness of surroundings of the upper electrode 15 is 80 nm, and therecess is formed such that the center portion has a thickness of 10 nm.

The low heat conductivity material which is provided for the depressedportion is SiO₂.

The insulating layer 16 is using Al₂O₃.

Although not shown, in respect to this embodiment, a comparative examplehaving the same layer structure except for the upper electrode 15 wasprepared.

In the comparative example, the low heat conductivity part 18 is notformed in the upper electrode 15, and the thickness of the upperelectrode 15 as a W layer is set to a constant 100 nm in the directionof the entire layer surface.

The device resistance in both the embodiment and the comparative exampleis 2 to 3 kΩ in a state of parallel magnetization.

FIG. 9A shows the calculated results of the relative temperature risesof both the comparative example and the embodiment when the memory layer14 generates heat uniformly.

The horizontal axis denotes the distance (nm) from the center of thelayer surface of the device. The vertical axis denotes the ratio oftemperature difference (Ts) and temperature difference (Te), wherein thetemperature difference (Ts) means a difference between the ambienttemperature and the temperature of the end portion of the device and thetemperature difference (Te) means a difference between the temperatureof the memory layer 14 and the surrounding temperature depending on thedistance (0±75 μm) from the center.

Here, the expression “ambient temperature” means the temperature of theenvironment which is the temperature at a distance sufficiently removedfrom the magnetic memory device 1, for example, room temperature. Also,the “the end portion of the device” in this example means thesurroundings of the circular part of the memory layer 14 as the layerhas a circular layer surface.

Because the memory layer 14 has a circular layer surface, thetemperature distribution is symmetrical about the center. The distance 0is the center of the circular memory layer 14 and the distance ±75 μm isthe end portion of the circle.

As shown in FIG. 9A, although the temperature difference ratio of Te/Tsat the center portion in the direction of the layer surface in thecomparative example is about 1.2, the ratio in embodiment is expected tobe about 1.6. That is, it is possible to obtain a prominent temperaturerise in the vicinity of the center portion of the later surface of thememory layer 14 with respect to the surroundings (the end portion of thedevice).

In the structure of the embodiment and the comparative example describedabove, FIG. 9B shows the memorization voltage (a voltage necessary toreverse the magnetization) when the temperature characteristics of thecoercive force of the memory layer 14 are changed. The vertical axisdenotes memory voltage Vc, and the horizontal axis denotes the ratio ofthe coercive force (Hc 200° C.) at a temperature of 200° C. which is ageneral temperature during memorization and the coercive force (Hc 23°C.) at a room temperature of 23° C.

Black circle denotes a sample of the embodiment, and white circledenotes a sample of the comparative example. In each structure, theratio (Hc 200° C./Hc 23° C.) of the coercive force can vary by adjustingeach layer and the entire thickness of the layers of the alternatelylaminated layers of Co/Pd of the memory layer 14. The memory layer 14was adjusted in order to produce a coercive force of 400 to 600 Oe at atemperature of 23° C.

The memory voltage Vc shows the average of both positive and negativepolarities at a pulse width of about 10 ns.

Meanwhile, regarding the coercive force of the memory layer 14 of thedevice, when there is no external magnetic field due to a leakagemagnetic field from the reference layer 12, since a difference occurs incoercive force from parallel to semi-parallel, calculation was performedwhile applying a certain external magnetic field to cancel the leakagemagnetic field from the reference layer 12.

In the comparative example, if the ratio of the coercive force attemperatures of 200° C. and 23° C. is lowered, the voltage for reversalis reduced little by little, but with only a small amount of change.

Meanwhile, in the embodiment, if the ratio (Hc 200° C./Hc 23° C.) of thecoercive force at temperatures of 200° C. and 23° C. is 0.7 or less, thevoltage for reversal is reduced rapidly.

Therefore, in a structure of the embodiment, it is possible to realizenonvolatile memory operable at high speed with low current by making theratio (Hc 200° C./Hc 23° C.) of coercive force of the memory layer 140.7 or less.

Meanwhile, in order to hold true when the coercive force is 0 at atemperature of 200° C., although it is possible for the ratio ofcoercive force to be 0, the reason for the large deterioration in thecoercive force at 200° C. is that the variance is high at roomtemperature. For this reason, it is preferable to have a stable coerciveforce at around room temperature, and it is more preferable to make theratio of the coercive force 0.3 or greater.

Accordingly, although it is preferable to make the ratio (Hc 200° C./Hc23° C.) of coercive force 0.7 or less, it is more effective to make it0.3 or more and 0.7 or less.

As explained hereinbefore, the magnetic memory device 1 of theembodiment includes a memory layer 14 having a vertical magnetization onthe layer surface and changing the direction of the magnetizationaccording to information, and a reference layer 12 provided to thememory layer 14 through the nonmagnetization layer 13 and having avertical magnetization on the layer surface while having a fixedmagnetization direction. Furthermore, information is memorized by themagnetization reversal of the memory layer 14 caused by a spin torquegenerated when current flows between layers made from the memory layer14, the nonmagnetization layer 13, and the reference layer 12.

In this structure, it is preferable to make the coercive force of thememory layer 14 at a memorization temperature 0.7 times or less thanthat at room temperature. Furthermore, it is preferable to make the heatconductivity of the center portion of the upper electrode 15 in thedirection of the layer surface formed on one side of the memory layer 14less than that of the peripheral part.

By structuring like this, effective magnetization reversal is realizedby the temperature rise at the center portion of the memory layer 14,and a current for memorization and a time for memorization can bereduced.

In particular, as shown in FIGS. 4A to 4D and FIGS. 6A to 6E, thethickness of the cross section of the central part of the upperelectrode 15 is thinner than that of the circumference part of the upperelectrode 15 so that the cross section is approximately a concave-likeshape, and an insulator of low heat conductivity is filled into thedepressed portion formed in the center portion. Alternatively, as shownin FIGS. 5A to 5E and FIG. 7A to 7F, the upper electrode 15 is formed ina tube shape, and the internal part of the tube shape is filled with aninsulator of low heat conductivity. Accordingly, as well as realizingthe embodiment of the magnetic memory device 1 with comparative ease, itis effective to reduce the current for memorization and the time formemorization.

The embodiment has been explained, but the structure of the magneticmemory device 1 or the method for manufacturing the magnetic memorydevice 1 is not restricted to the embodiment. The materials of thememory layer 14, the nonmagnetization layer 13, the reference layer 12,the upper electrode 15, the low heat conductivity part 18, or the like,and the shape and the like of the upper electrode 15 for providing thelow heat conductivity part 18 are variously considered.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2010-177104 filed in theJapan Patent Office on Aug. 6, 2010, the entire contents of which arehereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A magnetic memory device comprising: a memorylayer having a vertical magnetization, the memory layer being on a layersurface, the direction of magnetization of the memory layer changingaccording to information recorded therein; a nonmagnetization layer; areference layer provided against the memory layer with thenonmagnetization layer therebetween, the reference layer being a basisof information while having a vertical magnetization on the layersurface; and an electrode formed on one side of the memory layer in thedirection of the layer surface, wherein, the memory device memorizes theinformation by reversal of the magnetization of the memory layer by aspin torque generated when a current flows between the memory layer, thenonmagnetization layer and the reference layer, a coercive force of thememory layer at a memorization temperature is 0.7 times or less than acoercive force at room temperature, and a heat conductivity of a centerportion of the electrode formed on one side of the memory layer in thedirection of the layer surface is lower than a heat conductivity ofsurroundings thereof
 2. The magnetic memory device according to claim 1,wherein the electrode is formed to substantially have a concave crosssection in which a thickness of the center portion thereof is thinnerthan that of the surroundings thereof, and an insulator of low heatconductivity is filled into a depressed portion formed at the centerportion.
 3. The magnetic memory device according to claim 1, wherein theelectrode is formed to have a tube-like shape, and an inner part of thetube-like shape is filled with an insulator of low heat conductivity. 4.A method of manufacturing a magnetic memory device comprising a memorylayer having a vertical magnetization on a layer surface the directionof the magnetization of which changes according to information recordedtherein; and a reference layer provided against the memory layer with anonmagnetization layer therebetween, and being a basis of informationwhile having a vertical magnetization on the layer surface, the memorydevice memorizing the information by reversal of the magnetization ofthe memory layer by a spin torque generated when a current flows betweenthe memory layer, the nonmagnetization layer and the reference layer,the method comprising: forming a layer structure having at least thereference layer, the nonmagnetization layer and the memory layer on anelectrode of the reference layer side, and making the layer structurehave a coercive force of the memory layer at a memorization temperaturebeing 0.7 times or less than a coercive force at room temperature; andforming another electrode which is filled with an insulator of low heatconductivity, and of which a heat conductivity of a center portion inthe direction of the layer surface is lower than a heat conductivity ofits surroundings, on one side of the memory layer.